Numerous fields of application require the deployment of heavy loads to/from a location of interest, including building, construction, mining, oil and gas, etc. One such application involves the deployment of sub-sea hardware in very deep water, e.g., at depths of 1000 m and greater. Deep water deployment of sub-sea hardware is particularly associated with the oil and gas industry. Examples of such sub-sea hardware include manifolds, templates, processing modules and wellhead systems. Assemblies of this type can weigh hundreds of tonnes. Similarly, extreme loads may be encountered when lifting or lowering a pipeline or section of pipeline to or from the seabed during installation and/or maintenance.
Deep water deployment systems including cranes employ a variety of mechanisms and typically include traction systems to move payloads via load-bearing spoolable media, such as metal, synthetic or natural fibre cables, wires and ropes. Traction systems include a drum winch around which a spoolable medium is wound, wherein rotation of the drum permits spooling of the medium.
In some species of winch the drum acts to store the spoolable medium, with the medium be arranged in single or multiple wraps and layers between end flanges of the drum. In such winch species, however, the spoolable medium may be subject to significant radial crushing forces, particularly in circumstances where large payloads are involved and thus significant tensions are applied to the spoolable medium. Further, in some applications it may be necessary to store the medium in a high tension state, which may reduce the life span of the medium through fatigue, excessive strains, hysteresis and the like. Furthermore, storage of the spoolable medium on a drum typically requires the use of complex fleeting arrangements to ensure that the medium is arranged in suitable wraps and layers.
In other species of winch the drum is used only to apply a force to a spoolable medium, with the spoolable medium being stored separately, for example in a basket, on a separate spool or the like. The force applied by the drum is typically either a pulling force to pay in a spoolable medium, or a controlled releasing force to permit controlled paying out of a spoolable medium while under load, for example while connected to a payload. In such winch species, which may include capstan or windlass winches, an intermediate portion of a spoolable medium is wrapped around the drum a number of times such that an outboard side of the spoolable medium extends from the drum to engage a payload, and an inboard side of the spoolable medium extends to storage. Under loaded conditions the drum functions to reduce the tension in the spoolable medium from a high tension condition in the outboard side, to a lower tension condition in the inboard side of the spoolable medium, thus permitting the spoolable medium to be stored in a favourable low tension state. In view of this tension reduction functionality, such winch species are often called detensioning units. In use, the drum establishes a tension gradient in the spoolable medium, which may be defined by the capstan friction equation:
            T      1              T      2        =      e          μ      ⁢                          ⁢      θ      wherein:                T1=outboard tension        T2=inboard tension        μ=co-efficient of friction between the spoolable medium and the drum or contact surface        θ=angle of contact with the drum (e.g., one wrap is 2π radians)        
The traction winch may include multiple sheaves over which the rope is drawn both to provide adequate traction and to progressively unload the rope before it is passed to the storage take up reel at low tension. An example of such traction winch is disclosed in U.S. Pat. No. 6,182,915 (ODIM HOLDING ASA), which is incorporated herein by reference, in which the multiple sheaves are separately powered in a manner to prevent the cable from being damaged by slipping as it unloads. Another example is disclosed in International Patent Application Publication No. WO 2011/121272 (PARKBURN PRECISION HANDLING SYSTEMS LTD), which is incorporated herein by reference, in which two traction winch drums configured to rotate about respective first and second axes of rotation which are inclined relative to each other. The relative inclined alignment of the first and second axes of rotation of the drum assemblies permits the respective drum contact surfaces to cooperate to manipulate an associated spoolable medium to follow a predefined path, such as a predefined helical path.
When steel is used as a spoolable medium, the deployment of very large loads of the type described above requires the use of very large steel wire ropes or cables. However, especially at great depths, the weight of the steel itself becomes a significant problem. Not only does this impose tremendous loads on the lifting system but also, beyond a certain depth, it becomes impossible to make a wire rope large enough to support its own weight without exceeding its safe working loads, let alone the weight of the equipment to which it is attached.
In order to reduce the weight of the spoolable media used in very deep water applications, synthetic fibre ropes have been adopted. Synthetic fibre ropes typically exhibit near neutral buoyancy and therefore minimal added weight, even when working at great depths. Such ropes can be made from a variety of synthetic fibres. Ultra High Molecular Weight Polyethylene (UHMWPE) fibre rope has proven especially successful due to its high strength to weight ratio and low elongation under loads. For example, suitable UHMWPE fibre ropes are available under the Dyneema® trademark of DSM, The Netherlands.
Although synthetic fibre ropes offer a viable solution for deep water deployment, and are vastly superior to steel wire rope in many respects, they nevertheless present special challenges of their own, especially when used in larger diameters. In particular, when used with traction winches, synthetic fibre ropes typically require larger diameter sheave wheels than do wire ropes. A number of reasons for this may include (among others) the susceptibility of individual fibres to fracture when bent and also the relative inability of the fibre material to shed heat due to its low thermal conductivity, which can in turn lead to heat build-up and damage to the fibres in the core of the rope. As a result, it has been determined that the practical minimum “D:d” diameter ratio for using large synthetic fibre ropes on traction winch systems is approximately 30:1, wherein “D:d” represents the ratio between the diameter of the sheave wheel and the diameter of the rope. Current research focusing on loads of 250Te indicates that a synthetic fibre rope having the requisite capacity (including industry established safety margins) will have a diameter on the order of 140 mm. Based on the minimum 30:1 ratio, the corresponding minimum sheave diameter is approximately 4.2 m, which is very large. A 750Te system would require a proportionately larger rope, to the point where the sheave wheels and associated machinery would be prohibitively large. Not only is the cost of such equipment very high, but it is compounded by the need to use a larger vessel and a larger crew, to the point where feasibility is drawn into question.
Furthermore, very large diameter synthetic ropes present additional problems. In particular, ropes do not scale well and suffer a loss of strength translation efficiency in their larger sizes. Furthermore, it has been found that, even when using optimally-sized sheaves, the larger the rope the lower the number of bend cycles it is able to sustain before failure. Although the reasons for this are not entirely clear, and without wishing to be bound by theory, it is believed that this may be primarily due to the mass of material involved and the impact of the heat and abrasion generated by the greater number of crossover points within the rope structure, complicated by the insulation efficiency of the fibre material. Yet another difficulty is that splices or other repairs in large-size synthetic ropes increase diameter, which makes it very difficult for these to pass through the grooves of conventional sheave wheels, particularly on the leading sheave wheel where the rope is under extreme tension.
Systems using multi fall arrangements have been used in the past to seek to overcome some of the limitations cited above, and have been used with both steel wire and fibre rope systems. However, although this technique overcomes the need for large diameter ropes, some limitations of this approach include reduction of deployment speed by a factor proportionate to the number of falls in the moving block. This creates a significant increase in deployment time and hence results in a high cost impact when deploying payloads in great water depths. This also creates difficulty achieving sufficient speed in the lifting line when employing active heave compensation required for decoupling the vessel motions from the payload during deployment.
Systems using multiple separate drum winch systems with single lift lines connected to the payload have been used with some success to overcome these issues. However, the challenge of controlling multiple systems and balancing high tension loads in each of the lifting wires is a significant challenge, and the risks involved if precise control is not maintained between the separate winch systems make this technique difficult to implement.
Various arrangements of multiple ropes or cables combined with one or more drums are disclosed in EP 1 460 025 (Strödter), U.S. Pat. No. 605,937 (Turner), U.S. Pat. No. 6,042,087 (Heinemann), U.S. Pat. No. 4,600,086 (Yamasaki et al.), JP 11-011882 (Mitsubishi), JP 07-196288 (Japan Steel Works), SU 412133 (Leningrad Lengidrostal), and CN 201220899 Weihua Group).